1. Field of the Invention
The invention relates to a semi-creeping discharge type spark plug in which a spark discharge gap is formed by an air-gap and a creeping spark discharge gap through which spark discharges run along a front end surface of an insulator.
2. Description of Prior Art
As shown in FIG. 6, a semi-creeping discharge type spark plug (J) has been known in which a cylindrical metal shell 1 and an insulator 2 are provided, the latter of which has an axial bore 22 and is placed in the metal shell 1 so that a front end of the insulator 2 extends from a front end surface 11 of the metal shell 1. Within the axial bore 22, a center electrode 3 is placed, a front end surface 31 of which is located at a level substantially the same as the front end surface 23 of the insulator 2. L-shaped ground electrodes are provided which is welded to the front end surface 11 of the metal shell 1 as designated at numeral 4. In this situation, the front end surface 31 of the center electrode 3 is generally in flush with a forward edge portion 42 of a front end surface 41 of the ground electrode 4. Upon applying a high voltage across the electrodes 3, 4, spark discharges creep along the front end surface 23 of the insulator 2.
In a provisional publication No. 0765017 published on Mar. 26, 1997 under EPO, a semi-creeping discharge type spark plug similar to that of FIG. 6 has been disclosed which however remains silent about a geometrical dimensional relationship between the front end surface of the insulator and the forward edge portion of the front end surface of the ground electrode. Upon considering the purposes of the invention disclosed in the provisional publication No. 0765017, the publication puts an emphasis on a prevention of the channeling phenomenon rather than an avoidance of the soot fouling to insure an extended service life. On the contrary, the present invention makes much of preventing the soot fouling even though permitting the channeling phenomenon in a tolerable degree.
As well known for those versed in the art, this type of the spark plugs are, in fact, superior to a general air-gap type spark plug in the point of fouling resistance because the formers are to burningly evaporate the carbon-related deposit collected on the front end surface of the insulator.
In those semi-creeping discharge type spark plugs, it is, however, recognized that the insulation resistance reduces due to the carbon-related deposit (FIG. 9) when the fouling resistance experimental test was carried out under very cold conditions (-15.degree. C.) in conformity with a predelivery pattern in FIG. 4 as described in detail hereinafter. Besides insuring a desirable fouling resistant property, it has generally been demanded to impart a good heat resistant property to a semi-creeping discharge type spark plug without inviting unfavorable channeling phenomenon.
Therefore, it is a main object of the invention to provide a semi-creeping discharge type spark plug which is capable of concurrently insuring a good heat resistance and fouling resistance so as to maintain a desirable insulation resistance for an extended period of time.